A mechanical apparatus which performs movements similar to motions of a human being using mechanical or magnetic actions is called “robot”. It is said that the word “robot” originates from a word ‘ROBOTA’ (slave machine) of a Slavic language. Here in Japan, robots began to be popularized at the end of the nineteen sixties. Most of them, however, were industrial robots such as manipulators or transport robots intended for automation and unmanning of manufacturing works in a factory.
Recently, research and development regarding legged mobile robots such as pet type robots which copy body mechanisms and motions of animals that perform four-leg walking like a dog or a cat, or robots (humanoid robot) called “human-type” or “humanoid” robots which are designed by modeling body mechanisms and motions of animals which perform bipedal upright walking such as a human being have proceeded, and also expectation that they be placed into practical use is increasing.
The significance in research and development of legged mobile robots of bipedal locomotion type called human-like or humanoid robots can be grasped, for example, from the following two points of view.
One of them is a human scientific point of view. In particular, a mechanism of a natural motion of a human being beginning with walking can be clarified in an engineering sense through a process that a robot having a structure similar to the lower limbs and/or the upper limbs of a human being is produced and a controlling method for the robot is devised to simulate a walking motion of a human being. It is expected that results of such research can be fed back significantly to the progress in various other research fields which handle a movement mechanism of a human being such as the human engineering, rehabilitation engineering or sports engineering.
Another point is development of a robot for practical use which supports the life of a human being as a partner of the human being, that is, which supports human activities in dwelling environments and other various scenes in everyday life. It is necessary for a robot of the type just described to learn a method of adaptation to human beings having individually different identities and to environments while being taught by human beings in various phases of life environments of human beings. It is considered that, in this instance, if the robot is a “human type” robot, that is, if the robot has a same configuration or a same structure as that of the human being, then the robot functions effectively in smooth communication between a human being and the robot.
For example, in such a case that it is tried to actually teach to a robot a method of passing through a room while bypassing an obstacle which must be kept off, where the robot which is an object of teaching is a bipedal locomotion robot having a similar configuration to that of a user (operator), it is much easier for the user to teach the robot and also it is easier for the robot to learn than where the robot has a structure quite different from the user such as a crawler type robot or a four-legged robot (refer to, for example, TAKANISHI, “Control of a Bipedal locomotion Robot”, the Kanto Branch of the Society of Automotive Engineers of Japan <Koso>, No. 25, 1996 APRIL).
A great number of proposals have been made for a technique for posture control or stable walking relating to a robot of the type which performs legged movement by bipedal locomotion. The stable “walking” here can be defined as “traveling without tumbling through use of the legs”.
Posture stabilization control of a robot is very important in order to prevent tumbling of a robot. This is because tumbling of a robot signifies interruption of a work being performed and considerable labor and time are required until the robot stands up uprightly from the tumbling state and resumes the work. Above all, there is the possibility that the tumbling may critically damage the robot body itself or also damage a substance with which the tumbling robot collides. Accordingly, in research and development of legged mobile robots, posture stabilization control upon walking or upon any other legged operation is considered one of the most significant technical subjects.
Upon walking of a robot, the gravity and the force of inertia and moments of them originating from the gravity and an acceleration generated by the walking movement act from the walking system of the robot upon the road surface. According to the “d'Alembert's principle”, they are balanced with a floor reactive force and a floor reactive force moment as a reaction from the road surface to the walking system. As a consequence of mechanical inference, a point at which the pitch axis moment and the roll axis moment are zero, that is, a “ZMP (Zero Moment Point)”, is present on or on the inner side of a side of a support polygon formed by landed points (contact points) of the soles and the road surface.
Most of proposals regarding posture stabilization control of legged mobile robots and prevention of tumbling upon walking uses the ZMP as a criterion for determination of the stability of walking. Production of a bipedal locomotion pattern based on the ZMP criterion is advantageous in that a sole landing point can be set in advance and it is easy to take a kinetic restriction condition of the sole according to a shape of the road surface into consideration. Further, to adopt the ZMP as a stability determination criterion does not signify to handle not a force but a trajectory as a target value on motion control, and therefore, it technically raises the feasibility. It is to be noted that a concept of the ZMP and application of the ZMP to a stability determination criterion for a walking robot are disclosed in Miomir Vukobratovi'c, “LEGGED LOCOMOTION ROBOTS” (Ichiro KATO et al., “Walking Robot and Artificial Feet”, the Nikkan Kogyo Shimbun, Ltd.).
Normally, a bipedal locomotion robot such as a humanoid robot is higher in the position of the center of gravity and narrower in the ZMP stable region upon walking than a four-legged walking robot. Accordingly, the problem of a posture variation caused by a variation of the road surface condition is significant particularly with a bipedal locomotion robot.
Several proposals are already available which use the ZMP as a posture stability determination criterion for a bipedal locomotion robot.
For example, a legged mobile robot disclosed in Japanese Patent Laid-Open No. Hei 5-305579 performs stable walking by making a point on a floor at which a ZMP is zero coincide with a target value.
Meanwhile, another legged mobile robot disclosed in Japanese Patent Laid-Open No. Hei 5-305581 is configured such that a ZMP is positioned in the inside of a supporting polyhedron (polygon) or, upon landing or upon taking off, the ZMP is positioned at a position having at least a predetermined margin from an end portion of the support polygon. In this instance, even if the legged mobile robot is subject to some disturbance, it has a margin for the ZMP by the predetermined distance, and the stability of the body upon walking is improved.
Further, Japanese Patent Laid-Open No. Hei 5-305583 discloses that the walking speed of a legged mobile robot is controlled depending upon a ZMP target position. In particular, walking pattern data set in advance is used, and a leg part joint is driven so that a ZMP may coincide with a target position whereas a slope of the upper part of the body is detected and the discharging rate of the set walking pattern data is changed in response to the detection value. If the robot steps on an unknown concave or convex place and tilts forwardly, then the posture thereof can be restored by raising the discharging rate. Further, since the ZMP is controlled to the target position, there is no trouble even if the discharging rate is changed within a double support phase.
Japanese Patent Laid-Open No. Hei 5-305585 discloses that the landing position of a legged mobile robot is controlled in accordance with a ZMP target position. In particular, the legged mobile robot disclosed in the patent document mentioned detects a displacement between a ZMP target position and an actually measured position and drives one or both of the leg parts so that the displacement may be canceled. Or, the legged mobile robot detects a moment around a ZMP target position and drives the leg parts so that the moment may be reduced zero. The legged mobile robot thereby achieves stabilized walking.
Japanese Patent Laid-Open No. Hei 5-305586 discloses that a tilting posture of a legged mobile robot is controlled in accordance with a ZMP target position. In particular, a moment around a ZMP target position is detected, and if a moment appears, then the leg parts are driven so that the moment may be reduced to zero thereby to achieve stabilized walking.
Posture stabilization control of a robot which adopts a ZMP as a stability determination criterion basically resides in search for a point at which a moment is zero on or on the inner side of a side of a support polygon formed from landed points of the soles and the road surface.
As described hereinabove, for a legged mobile robot, possible much effort has been made to prevent the robot from tumbling during walking or during execution of some other motion pattern by taking such a countermeasure as to introduce a ZMP as a posture stabilization criterion.
Naturally, the state of tumbling of a robot signifies interruption of a work being performed by the robot and considerable labor and time are required until the robot stands up uprightly from the tumbling state and resumes the work. Further, there is the possibility that, above all, the tumbling may critically damage the robot body itself or also damage a substance with which the tumbling robot collides.
Although the possible best posture stabilization control is performed in order to prevent tumbling of a robot, the robot may still lose its stability in posture because of some defect in control, some unpredictable external factor (such as, for example, accidental collision with another substance, a road surface situation such as a projection or a depression on the floor, appearance of an obstacle or the like) to such a degree that it cannot be supported only with the movable legs thereof, resulting in tumbling.
Particularly in the case of a robot which performs bipedal legged traveling such as a human-type robot, since the position of the center of gravity is high and an uprightly standing stationary state itself of the robot is originally instable, the robot is likely to tumble. If the robot tumbles, then there is the possibility that critical damage may be applied to the robot itself or to the other party side with which the robot collides by the tumbling.
For example, Japanese Patent Laid-Open No. Hei 11-48170 discloses a control apparatus for a legged mobile robot by which, when the legged mobile robot is in a situation wherein it seems to tumble, the damage which may be given to the robot by the tumbling or the damage to the other party side with which the robot may collide upon the tumbling can be reduced as far as possible.
However, the patent document merely proposes control by which the center of gravity of the robot when landing upon tumbling is merely lowered, but does not make such an argument that, in order to minimize the possible damage when the robot actually tumbles, in what manner the entire body including not only the leg parts but also the body and the arm parts should operate.
In the case of a legged mobile robot of the uprightly standing walking type, a posture which makes a reference when a movement of a body such as walking is taken into consideration is a standing posture in which the robot stands uprightly with the two feet. For example, a state in which the robot is most stable among various standing postures (that is, a point at which the instability is in the minimum) can be positioned as a basic standing posture.
Such a basic standing posture as just described requires generation of torque by joint axis motors for the leg parts and so forth by execution of posture stabilization control and control instruction. In other words, in a no-power supply condition, no standing posture is stable. Therefore, it is considered preferable that the robot starts activation thereof from an on-floor posture in which the robot is physically most stable such as a supine posture or a prone posture.
However, even if power supply to the robot in such an on-floor posture as just described is made available, if the robot cannot stand up autonomously, then an operator must give a hand to lift the body, which is cumbersome to the operator.
Further, when the robot once assumes a standing posture and performs walking or some other autonomous legged operation, it basically makes an effect to travel using the legs without tumbling. However, the robot may sometimes tumble unfortunately. “Tumbling” is inevitable when a robot operates under dwelling environments of human beings which involve various obstacles and unforeseeable situations. In the first place, a human being also tumbles. Also in this instance, it still is cumbersome if an operator must give a hand to lift the body.
If the robot cannot stand up by itself every time it assumes an on-floor posture, then it cannot be operated in unmanned environments after all, and the operation lacks in self-conclusion. Thus, the robot cannot be placed into fully self-contained environments.